1. Field of the Invention
The present invention relates to an optical exposure system for a color video printer utilizing a liquid crystal display device (hereinafter, referred to as a "LCD") and more particularly, to an optical exposure system for a color video printer which is capable of obtaining a uniform irradiation of light with respect to red, green and blue LCD panels.
2. Description of the Prior Art
In general, various types of exposure systems for converting an electrical image into an optical image and exposing the optical image to a photosensitive medium are well known in the art. Recently, exposure systems utilizing a liquid crystal display television technique are widely used.
In such an exposure system, there has been involved some problems in that it takes relatively many times in the exposing procedure, thereby causing the printing operation not to be carried out quickly.
To obviate such a problem, the applicant of this invention has filed a Korean Patent Application No. 5794/1991 entitled "An Exposure System for a Color Video Printer", in which an exposing unit using liquid crystal display panels for red, green and blue colors having the same size as a screen is provided. In this exposure system, the exposing operation is sequentially carried out for red, green and blue colors while moving a photosensitive medium over the liquid crystal display panels so that the printing time is reduced and the high picture resolution may be obtained.
The exposure system mentioned-above is illustrated in FIGS. 1 to 3, which comprises a casing 30, liquid crystal display panels 22, 23 and 24 for red, green and blue colors disposed at the upper portion of the casing 30 and commensurate with a screen, a liquid crystal exposure unit 20 including light sources 26, 27 and 28 each for radiating a light to each of the liquid crystal display panels 22, 23 and 24, and a photosensitive medium supporting unit 10 disposed on the liquid crystal display pannels 22, 23 and 24 for directing the photosensitive medium which is transferred from a supply roll 2 to take-up rolls 3 and 3' to be in spaced with the liquid crystal display panels 22, 23 and 24 at a predetermined gap (d).
The exposure unit 20 is constituted such that a transparent support member 25 is fixed at the upper inner part of the casing 30 and on the transparent support member 25, the liquid crystal display panels 22, 23 and 24 are positioned. On the upper surface of the panels 22, 23 and 24, a transparent spacer member 21 is disposed. At the lower side of the liquid crystal display panels 22, 23 and 24, light sources 26, 27 and 28 and a reflective plate 29 for reflecting a light from the light sources 26, 27 and 28 toward the panels 22, 23 and 24 are disposed.
The photosensitive medium supporting unit 10 includes a solenoid 11, a support bar 12 operably connected to the solenoid 11 and provided with lower extensions, a pair of lever members 15 and 16 connected at one ends thereof to each other by means of a pin 17 and at other ends thereof to the lower extensions of the support bar 12 by means of connecting pins 18 and 19, and a pair of rolls 13 and 14 mounted to outer ends of the lever members 15 and 16 for pressing the photosensitive medium 1 against the upper surface of the transparent spacer member 21 so that the lever members 15 and 16 may be rotated upwardly about the pin 17 and the rolls 13 and 14 may be moved left or right, thereby causing the photosensitive medium 1 to be in close contact with the transparent spacer member 21 in the exposing operation or to be spaced apart from the spacer member 21 when the exposing operation is not carried out.
Hereinafter, the operation of the conventional exposure system will be described.
When the photosensitive medium 1 is located over the liquid crystal display panel 22 for red color by the operation of the supply roll 2 and the take-up rolls 3 and 3' and then a signal voltage for red color is applied to the liquid crystal display panel 22 for red color and the light source 26 is turned on, as shown in FIG. 3A, an optical image is formed in accordance with the red color signal and the optical image is irradiated to the photosensitive medium 1. At this moment, it takes, in general, about 5-15 seconds in the above-mentioned procedures.
Upon completion of the exposing operation for red color, the light source 26 is turned off and the photosensitive medium 1 is transferred ever the liquid crystal display panel 23 for green color, as shown in FIG. 3B, by means of the supply roll 2 and the take-up rolls 3 and 3'. Then, the liquid crystal display panel 23 for green color is driven by a green color signal voltage and the light source 27 is turned on so that a green color image is formed to expose the portion that the red color image has been exposed on the photosensitive medium 1.
Thereafter, the exposing operation for blue color is also carried out in the same manner as described above.
In such an exposing operation, the solenoid 11 is turned off so that the supporting bar 12 descends, thereby the lever members 15 and 16 are pressed down about the connecting pin 17. As a result, the rolls 13 press down the photosensitive medium 1 against the transparent spacer member 21 throughout the whole surface. Thus, a gap (d) which is a thickness of the spacer member 21 is maintained between the photosensitive medium 1 and the panels 22, 23 and 24 throughout the whole surface.
On the other hand, upon completion of the exposing operations, the solenoid 11 is turned on to pull upwardly the supporting bar 12 so that the rolls 13 and 14 are raised up, thereby allowing the photosensitive medium 1 to be easily transferred without any friction.
In such a conventional exposure system for a color video printer, although it is possible to shorten the time required in the exposing operation and to obtain a high resolution by use of liquid crystal display panels commensurate with the whole screen, there have been involved some problems in that since the light sources are fixed in their positions, the number of the light sources are increased in proportion to the size of the screen in case that the screen has a large size.
Moreover, since the distribution of the light which is irradiated directly and indirectly to the liquid crystal display panels for red, green and blue colors are not uniform, the picture quality becomes deteriorated. In addition, the light from the light sources are not irradiated to a desired liquid crystal display panel but to all the panels, so that a light loss occurs and also an unnecessary exposing may be carried out to the photosensitive medium by a light which has been irradiated to other panels.